The axial thrust of a gas turbine is determined by two distinct types of loads. The aerodynamic load exerted on the turning blades of the compressor and the turbine. There are also pressure forces which act on the rotor in the axial direction. The total resulting force is applied to the thrust bearing of the gas turbine. Optimizing the bearing size can maximize the bearing life while minimizing the power loss due to the bearing resistance. To properly size the bearing both the aerodynamic and pressure loads must be calculated. Both of these load types vary with the operating parameters. The aerodynamic loads can be calculated in a relatively straight forward manner due to the consistent and repeating geometry that is being acted on. The pressure forces are more complicated since the varying geometry of the rotor attachment structures from stage to stage. Therefore the pressure generated in each cavity formed by the rotor and stator disks must be considered separately. Unlike the aerodynamic forces which all act in the same direction, the pressure forces can act either in the gas flow direction or opposite to the gas flow direction depending on the position of the cavity with respect to the rotor disk. Therefore, the sum of all the pressure forces can have either a net positive or net negative load on the bearing.
Two methods have been used for calculating the thrust loads attributed to secondary air systems (the pressure forces). In the first method, either a one dimensional fluid flow software or a 3D computational fluid dynamics (CFD) software would be used to determine the average static pressure for the entire cavity formed by the rotating and stationary blade disks of a gas turbine engine. The vertical distance from the bottom of the cavity to the top is measured for each side that is rotating. The area of that surface is then manually calculated using the formula for the area of an annulus. This area is then multiplied by the static pressure calculated in the simulation tool to get the force applied on the bearing by that surface.
Assume the rotational axis of the gas turbine is horizontal and the gas flow direction is from left to right. If the surface is on the left side of the cavity, the force is considered negative. If it is on the right side then it is considered positive. This is then repeated for all the cavities in the engine and all the forces are summed up to determine the net force on the bearing.
This method is not accurate because the pressure can vary significantly across the span of the surface. Averaging the pressure inherently thus introduces an error. This method is not labor intensive however and is commonly used at least in the early design process.
The second method is similar to the first one except the vertical areas are broken up into smaller segments and the static pressure is determined based on simulation for each of the discretized segments. The vertical distance of each annular sector is measured and the area of the annulus is calculated. The static pressure is then multiplied by the area of the annulus to get the force on the annular area. The forces for all of the annuli for a particular surface are summed and the direction of the force is determined in the same manner as the first method.
The above process is then repeated for all vertical rotating surfaces for all the cavities in the engine and all the forces are summed up to determine the net force on the bearing. This method is more accurate than the first method since the pressures are averaged over a much smaller area. The tradeoff is it is more labor intensive. As such, the method is normally used once designs are nearly finalized.
Both of the aforementioned conventional methods require human involvement in determining the direction of the forces. Even the more accurate second method may not be accurate enough for some applications. It is desirable to develop a method that enables computers to determine force directions automatically and to compute the thrust loads more accurately.